Electronic ice bank control



Feb. 24, 1970 A. E. PARKER ETAL 3,496,733

ELECTRONIC ICE BANK CONTROL Filed May 1. 1968 66 Condenser INVENTORS. Ar'fhur E. Parker Rober'f E. Paffer'son I A ORNEYS.

States U.S. Cl. 62- 139 9 Claims ABSTRACT OF THE DISCLOSURE An ice bank control for a cup beverage dispensing machine employs a bridge network having two levels of sensitivity to provide a differential between turn-on and turn-off of the refrigeration system in order to prevent short cycling. The bridge network responds to changes in the resistivity of the water or ice slush as detected by a pair of spaced electrodes immersed in the water bath. When the refrigeration system is activated by the control, a third electrode is electrically inserted in the water bath by a triac which is also employed to make or break a control circuit for the refrigeration system to effect on and off cycling thereof to maintain the size of the ice bank between desired maximum and minimum limits.

In a cup beverage dispensing machine it is required that the ice bank for cooling the water be controlled within minimum and maximum size limits. Ice bank controls responsive to the change in resistance of the water as it freezes and melts are presently in use. However, it is necessary that practical ice bank controls operate with adequate differential to eliminate any tendency to short cycle the refrigeration system utilized to maintain the size of the bank between the desired limits.

The reduction in the resistance of the water between two spaced electrodes in the cold water supply tank within which the ice bank is maintained commonly provides a signal for starting the refrigeration system, the decrease in resistance being indicative that the ice or slush between the electrodes has changed to water. However, once refreezing commences it is necessary that the sensitivity of the control be changed to prevent short cycling of the refrigeration system. Furthermore, it is also desirable and has been the practice to electrically insert a third electrode with a requirement in the control logic that refreezing occur between both the first two electrodes and one of the latter and the third electrode before cutoff of the refrigeration system. This assures that a sizable new ice bank is formed before the control terminates the operation of the refrigeration system.

Heretofore, controls of the type outlined above have not at all times met desired performance limits consistently under all environmental conditions. Operational limits may be dependent upon line voltage, temperature, or broad parameters of semiconductor devices. Additionally, such controls customarily effect electrical insertion of the third electrode during freezing of the water by means of an additional set of contacts on the control relay of the refrigeration compressor. The current carried by these contacts is oftentimes so small that conductivity tends to be unreliable.

It is, therefore, the primary object of this invention to provide an improved ice bank control having increased reliability and less dependence upon environmental con ditions and component parameters.

As a corollary to the foregoing object, it is an important aim of the instant invention to provide an improved control as aforesaid which provides the necessary differential to prevent short cycling of the refrigeration system and also effects electrical insertion of a third reatnt sistance-sensing electrode without the use of relay contacts or other mechanical switching means.

Furthermore, it is a specific and important object of the invention to provide such an improved control wherein a bridge network is employed and is operable to provide the required differential, and wherein electronic switching is utilized to insert the third electrode simultaneously with activation of the control circuit for the refrigeration system to commence operation thereof and refreezing of the ice bank once it has reached the minimum permissible size.

In the drawing:

FIGURE 1 is a diagrammatic illustration of a cold water supply tank for a cup beverage dispenser, the control of the instant invention being shown schematically in association with the ice bank refrigeration system; and

FIG. 2 is an electrical schematic diagram of an equivalent circuit illustrating the operation of the bridge network of the control.

A cold water supply tank 10 has a lid 12 and is provided with a water inlet pipe 14 containing a valve which is operated by a float 16. The level of the water bath within tank 10 is illustrated at 18; an outlet pipe 20 extends from the bottom of tank 10 and furnishes cold water upon demand.

A refrigeration system is employed to maintain an ice bank 22 in the central portion of tank 10, the bank 22 being diagrammatically illustrated and shown surrounding an evaporator coil 24 that forms a part of the refrigeration system. A pair of lines 26 for the refrigerant eX- tend from the ends of coil 24 through lid 12 to a compressor 28, an expansion valve 30, and a condenser 32 arranged in the conventional manner. The compressor 28 is operated by an internal electric motor, the power leads to such motor being illustrated at 34 and 36. When the compressor 28 is in operation, it is apparent that water freezes around the coil 24 to form the ice bank 22. Manifestly, the size of the bank 22, during periods of light demand, could increase until a substantial portion or all of the water within tank 10 is frozen solid, unless the refrigeration system is cycled to maintain the size of the ice bank 22 within predetermined limits.

The limits are defined by mounting a probe in the water bath in the form of three spaced electrodes 38, 40 and 42. The electrode 38 is nearest the coil 24, the remaining electrodes 40 and 42 being spaced progressively further from coil 24 in a direction radially thereof. The resistivity of water varies substantially in accordance with its physical state. With a spacing between the electrodes 38 and 40 and the electrodes 40 and 42 of approximately /2 inch, the resistance between each of such pairs of electrodes in the presence of water in the liquid state is approximately 20,000 ohms, depending on the temperature of the water and the impurities it contains. As the water is cooled and begins to change to ice, this resistance will increase to approximately 100,000 ohms, again depending upon the impurity content. In the instant invention, this resistance variation of water is utilized to cycle the refrigeration system in a manner to be described hereinafter.

A pair of power terminals 44 and 46 are connected to a suitable source of alternating current such as a 60 cycle, volt supply. A step-down transformer 48 has a primary winding 50 connected across terminals 44 and 46, and a secondary winding 52 having a center tap 54. As will become clear, alternating current from secondary 52 is applied to the three electrodes 38, 40 and 42, and is selected for the energization of the electrodes to prevent electrolysis of the water. This is of particular significance in order to prevent deterioration of the electrodes by the plating phenomenon, which would occur with most materials if a direct current were applied.

A polarity sensitive device in the form of a monolithic Darlington amplifier 56 has a base input 58, a collector output connection 60, and an emitter connection 62, the latter being directly connected to the center tap 54. An input resistor 64 is connected across base 58 and emitter 62. Collector voltage is supplied by a lead 66 extending from the upper end of secondary winding 52 to a capacitor 68, a series connected diode 70 and resistor 72 connecting capacitor 68 to collector 60 to supply the latter with positive potential during alternate positive half cycles of the alternating current delivered by secondary 52.

A pair of series resistors 74 and 76 form a voltage divider and are connected across lead 66 and center tap 54. A lead 78 is connected at the junction of resistors 74 and 76 and extends to the inner electrode 38. A lead 80 is connected to the same junction by a resistor 82 and extends to the outer electrode 42. A lead 84 is connected to base 58 by a coupling capacitor 86 and extends to the intermediate electrode 40. A bypass diode 88 is connected across emitter 62 and base 58 and is poled in the opposite direction to the forward conduction direction of the input of amplifier 56.

An electronic switching component in the form of a triac 90 has its internal current path connected in series between lead 66 and a lead 92 that extends to the coil 94 of a control relay 96 for the compressor 28. A control circuit for the compressor 28 is formed by the lead 92 and a lead 98 connecting the other electrical side of the relay coil 94 to the lower end of the secondary winding 52. The control relay 96 includes a normally open relay switch 100 which is interposed in series with the power lead 36. The power leads 34 and 36 extend to power terminals 46 and 44 respectively.

The triac 90 has a control input or gate 102 connected to lead 66 by a resistor 104, and connected to the junction of capacitor 68 and resistor 72 by a resistor 106. The capacitor 68 is employed to store trigger current for the gate 102, and the resistor 106 lengthens the time constant.

A pair of resistors 108 and 110 are in parallel (via lead 92 and relay coil 94) between lead 98 and a lead 112 which is connected to the lead 84 extending to electrode 40. A lead 114 joins the interconnection of lead 92 and resistor 108 to a resistor 116 connected to the lead 80 that extends to electrode 42.

As mentioned hereinabove, operation of the compressor 28 must be cycled in order to control the size of the ice bank 22. The water bath and ice bank arrangement illustrated herein is particularly suitable for cup beverage dispensers where the demand for cold water is subject to extreme variations. For example, there may be long periods such as overnight in which the dispensing machine is not in use. Therefore, the ice bank 22 must be maintained under conditions where only occasional operation of the refrigeration system is required, and under other conditions such as heavy cold water demand where perhaps relatively constant operation of the system will be necessary to maintain the ice bank 22.

The equivalent circuit of FIG. 2 is instructive in the understanding of the operation of the instant invention to satisfy the above requirements. A bridge network is formed by various elements of the circuitry of FIG. 1, as will be appreciated with reference to the equivalent circuit. The bridge shown in FIG. 2 has four resistance arms designated R R R and R An AC source 118 is connected to the two input terminals of the bridge presented by the junctions of arms R and R and R and R These two input terminals are designated 66 and 98 respectively and correspond to the upper end of the transformer secondary 52 (lead 66) and the lower end of the transformer secondary 52 (lead 98) illustrated in FIG. 1. The output terminals of the bridge are designated 54 and 56 and, therefore, correspond to the center tap of the transformer secondary 52 and the base input of amplifier 56 respectively. Thus, resistance arms R and R are equivalent to 4 the transformer secondary 52 and the resistance arm R is equivalent to the parallel resistors 108 and 110.

The resistance arm R comprises the control arm of the bridge and is variable in resistance in accordance with the resistivity of the water between the electrodes 38 and 40 of the ice bank probe. For purposes of illustration, the bridge will be considered to be at a balanced condition when the resistance of R is approximately 100,000 ohms. The Darlington amplifier 56 is illustrated in FIG. 2 as a NPN transistor amplifier, its equivalent for purposes of explanation of the operation of the network. The collector of amplifier 56 in the equivalent circuit is connected through an output resistor to the bridge input terminal 66; therefore, the amplifier 56 is in condition for conduction only during alternate half cycles of the alternating current when terminal 66 is positive. The condition of the bridge at times when the collector of amplifier 56 is negative may thus be disregarded.

Assuming that the compressor 28 is not in operation and that the ice slush between electrodes 38 and 40 is thawing due to the effect of ambient temperature or demand or both, the resistance of R is decreasing and ultimately reaches a value where the bridge is out of balance to an extent to turn on the Darlington amplifier 56 or its single transistor equivalent in FIG. 2. It is assumed that the turn-on point corresponds to a resistance of R of approximately 30,000 ohms, which is 70,000 ohms less than the resistance in arm R needed for a balance condition. Thus, when input terminal 66 is positive, current flows from output terminal 58 to output terminal 54 across the base-emitter junction of the amplifier 56. When arn plifier 56 assumes its conductive state, an output appears across the collector output resistor (FIG. 2) and is utilized, as will be subsequently explained, to gate the triac 90. The diode 88 permits current flow between output terminals 54 and 58 when the bridge unbalances in the opposite direction at times when input terminal 66 is negative and input terminal 98 is positive.

Referring to FIG. 1, it may be seen that the Darlington amplifier 56 in its excited, conductive state provides a current path through the capacitor 68 to the center tap 54. Voltage is now applied to the gate 102 of the triac to render the latter conductive, the capacitor 68 serving to store trigger current for gate 102 during the alternate half cycles of the alternating current from transformer secondary 52 in which lead 66 is negative with respect to center tap 54 and amplifier 56 is nonconductive; therefore, the triac 90 remains on during both positive and negative half cycles of the alternating current. Turn-on of the triac 90 effects energization of relay coil 92 to close switch and place the compressor 28 in operation.

Additionally, when the triac 90 is rendered conductive, lead 114 is placed at substantially the same potential as lead 66 to increase the potential at electrode 42 with respect to the center tap 54. Furthermore, the potential at lead 112 (base input 58 decreases with respect to the center tap 54. Referring to FIG. 2, this change in the potential at lead 112 is equivalent to disconnecting arm R from bridge input terminal 98 and reconnecting R to a tap on R Thus, the voltage reference of the bridge out-put terminal 58 relative to the bridge output terminal 54 is now between terminals 54 and 98. This throws the bridge further out of balance and increases the current fiowing between the output terminals 58 and 54 and, therefore, through the input circuit of the amplifier 56. It will be appreciated that the amplifier 56 is driven harder and thus the sensitivity of the network is changed, requiring that the resistance R change to a substantially higher value in order to decrease the drive to the amplifier sufiiciently to return the same to its nonconductive state.

As the compressor 28 operates and the water or ice slush between electrodes 38 and 40 refreezes, the resistance between these two electrodes increases. It is assumed that a resistance of R of 70,000 ohms is required to return amplifier 56 to its nonconductive state. Thus, as the ice bank 22 grows the resistance of R ultimately reaches 70,000 ohms normally prior to the time that the resistance between electrodes 40 and 42 reaches 70,000 ohms, since the latter two electrodes are spaced the same distance apart as electrodes 38 and 40 but electrode 42 is further from the evaporator coil 24. However, it is to be remembered that the potential difference across electrodes 40 and 42 was substantially increased at the time the triac 90 was turned on. Therefore, a second, independent path for flow of excitation to the amplifier base input 58 now exists between electrodes 40 and 42. The values of resistors 74, 76, 82 and 116 are selected to set the increased potential difference across electrodes 40 and 42 at a level to require the resistance therebetween to also increase to 70,000 ohms before drive current for amplifier 56 is reduced to cutoff. The combined effect, therefore, of the change in the sensitivity of the bridge network and the increasing of the voltage on electrode 42 is to require that the resistance between both of the pairs of electrodes 3840 and 40-42 reach 70,000 ohms before amplifier 56 will return to its nonconductive state. Thus, it is assured that a sizable new ice bank 22 is formed.

When the amplifier 56 returns to its nonconductive state, the triac 90 is rendered nonconductive and relay coil 94 is de-energized. This deactivates the compressor 28 but the evaporator coil 24 will continue to refrigerate the ice bank 22 for a time after the compressor 28 is deactivated, resulting in a further increase in the resistance of the ice slush between the electrodes 38 and 40. The bridge network is now returned to normal and thus will balance at a resistance of R of approximately 100,- 000 ohms. Should this occur and the bridge become unbalanced in the opposite direction by a resistance of R of greater than 100,000 ohms, the amplifier 56 will not respond since negative potential will be applied to the diode 70 during the time that current flows from the base input 58 to the emitter connection 62. Thus, the amplifier 56 is phase sensitive to preclude spurious operation of the refrigeration system.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. Apparatus for controlling the size of an ice bank maintained in a water-containing region by a refrigeration system, said apparatus comprising:

a pair of spaced electrodes adapted to be disposed in said region for sensing the resistance of water between the electrodes;

means for supplying electrical energy;

a bridge network having an input coupled with said supply means, a pair of output terminals, and a control arm including said electrodes as a resistive element thereof and thereby being variable in resistance in accordance with changes in the resistivity of the water between the electrodes during freezing and melting;

electrically responsive switching means for controlling operation of said system, and including a device coupled with said output terminals and having a normal state and an excited state for cycling said system on and off,

said control arm effecting unbalancing of the network to an extent to excite the device and cause the latter to assume its excited state in response to a change in one direction of the resistance between said electrodes to a first predetermined value; and

circuit means coupled with said switching means and said network for further unbalancing the network, when the device assumes its excited state, to preclude return of the device to its normal state until the resistance between said electrodes changes in the opposite direction to a second predetermined value different than said first value, whereby to establish 6 a resistance differential in said control arm between turn-on and turn-off of said system.

2. Apparatus as claimed in claim 1,

said change in one direction of the resistance between the electrodes being a decrease in the resistance therebetween,

said change in the opposite direction of the resistance between the electrodes 'being an increase in the resistance therebetween,

said second value being greater than said first value.

3. Apparatus as claimed in claim 2,

and a third electrode spaced from said pair of electrodes and adapted to be disposed in said region for sensing the resistance of water between the third electrode and one electrode of said pair of electrodes,

said circuit means being coupled with said third electrode for establishing a sufficient potential difference across the third electrode and said one electrode, when said device assumes its excited state, to provide a path between said third electrode and said one electrode for flow of electrical excitation to said device to preclude return of the latter to its normal state until the resistance between said third electrode and said one electrode increases to said second value together with the resistance between said pair of electrodes, whereby to assure that the ice bank has sufficient size before freezing is terminated.

4. Apparatus as claimed in claim 1,

said device having input means for sensing current flow 'between said output terminals to effect a change of state of the device from the normal to the excited state when the network is unbalanced to said extent,

said circuit means effecting an increase in the current flow between said output terminals to apply greater drive to the input means of said device in response to said change of state thereof from the normal to the excited state.

5. Apparatus as claimed in claim 1,

said electrical energy comprising alternating current,

said device having polarity sensitive input means for sensing a unidirectional current flowing between said output terminals, and a polarity sensitive output connection coupled with said supply means and conditioned for operation of said device in its excited state only during alternate half cycles of said alternating current of a particular polarity, whereby the device undergoes a change of state from the normal to the excited state thereof only when said alternating current is of said particular polarity and said unidirectional current simultaneously flows between said output terminals, thus the device will not assume its excited state if the network should become unbalanced by a change in said opposite direction of the resistance between said electrodes.

6. Apparatus for controlling the size of an ice bank maintained in a water-containing region by a refrigeration system, said apparatus comprising:

first, second, and third spaced electrodes adapted to be disposed in said region for sensing the resistance of water between the first and second electrodes and the second and third electrodes, said resistance being variable in accordance with changes in resistivity of the water during freezing and melting;

means for supplying electrical energy;

a control circuit for said system coupled with said supply means;

electrically responsive switching means operably coupled with said circuit and including a device having a normal state and an excited state for controlling current flow in said circuit;

said device being provided with an operating input coupled with said second electrode;

said first electrode being coupled with said supply 7 8 7 means to cause electrical excitation to be delivered difference across the second and third electrodes when to the operating input of said device to effect a the device is in its excited'state. change of state of the latter from the normal to the 8. Apparatus as claimed in claim 6, excited state thereof when the resistance between said switching means further including an electrically said first and second electrodes decreases to a precontrollable electronic switching component in series determined value; and with said control circuit and said circuit means and circuit means interconnecting said control circuit and having a control input connected With the output said third electrode and, when said device assumes of said device, said excited state thereof, establishing a sufficient said component being responsive to changes of state potential difierence across the third electrode and 10 of said device for making and breaking said control said second electrode to provide a path therebecircuit to cycle the system on and off and simultween for flow of additional electrical excitation to taneously eflect the establishment of said potential said device to preclude return of the latter to its difference across the second and third electrodes normal state until the resistance increases between when the device is in its excited state. both the first and second electrodes and the third 9. Apparatus as claimed in claim 8, and second electrodes, whereby to assure that the said electrical energy comprising alternating current, ice bank has sufficient size before freezing is terniisaid switching component comprising a triac having nated. a gate presenting said control input. 7. Apparatus as claimed in claim 6, said switching means further including an electrically References C'ted controllable switching component in series with said UNITED STATES PATENTS control ClI'Clllt and SaId CII'CUIt means and responsive 2,506,775 5/1950 Calabrese to said device for maklng and breaking the control 3,298,191 1/1967 Burke 62 140 circuit to cycle the system on and off and simultaneously effect the establishment of said potential MEYER PERLIN, Primary Examiner 

